Loud popping noise in left channel

[troplin]

My S.E.X. 3.0 was running smoothly for about half a year, and now suddenly the left channel is running amok.

It started yesterday evening with loud popping noise in the left channel. Interestingly, the amp was already running for quite some time (but silent) and the pops started only when I started playing a movie on youtube. After a short time the pops disappeared so I thought that it was just a glitch in the movie or in the movie decoder.

Today, the amp was running the entire morning, without playing any music. Just when I started playing music, the pops were there again, now even louder. I didn't want to damage my speakers, so I turned the amp.
I unplugged the speakers and turned the amp back on. But now I could hear a high pitched squeaky "bubbling" noise from the amp; of course I turned it off again immediately.

Where do I start troubleshooting? I don't want to turn the amp on in that condition to take the voltage tests, I fear that this could damage it.

There's one thing that I noticed that might or might not be connected:
Some time ago (when the amp was still quite new), there was a strange glowing spot on the plate of the left tube. This happened only once and I never saw it since.
The glow was orange-purple and definitely on the plate itself. It was definitely not fluorescence, which also appeared (in both tubes) in the beginning, but was more like blue-purple and in the glass.
So, could this be a faulty tube?

[Paul Birkeland]

You'll need to turn the amp on and do the voltage measurements.

[Doc B.]

It sounds like either a connection has come loose or a tube is going bad. As PB says, you should do voltage measurements to start to isolate the problem.

[troplin]

Ok, I turned it on and after a few seconds, the left tube was sparking. It's probably safe to say that the tube has gone bad.
Should I replace both tubes at once?
Do you sell replacement tubes or can you recommend a trustworthy seller? I didn't expect the tube to go bad so fast so I don't have any spare tubes yet.

[Doc B.]

Swap tubes and see if the sparking follows the tube. That will help to clarify whether the problem is internal to the tube or somewhere else in the circuit.

[troplin]

Swap tubes and see if the sparking follows the tube.

Yes, it follows the tube.

Should I redo the voltage tests once I've got a replacement tube, to be sure that the damage is not caused by an error in the circuit?

And speaking of replacement tube:

Should I replace both tubes at once?
Do you sell replacement tubes or can you recommend a trustworthy seller? I didn't expect the tube to go bad so fast so I don't have any spare tubes yet.

[Paul Birkeland]

That's a good idea.  The 6FJ7 isn't a super tough tube to find, and any of the online tube vendors should be able to supply you with one.  They don't have to be a matched pair.

[troplin]

So I finally got my replacement tube(s) and just redid the voltage checks.
All the voltages were already on the high side when I built the kit but now almost all voltages are ~10% too high. It turns out that my mains voltage is 240V now, instead of the 228V-233V half a year ago.

Could this be the cause of the tube failure or is that unlikely?

Now, having such a high variance of input voltages, should I change the PT wiring to 240V or is it better to leave it at 230V as it is now?

[Paul Birkeland]

Yeah, I would go to 240V.  Running the amp with 240V on the 230V setting will affect tube life, but we are talking about getting maybe 5-10 years out of a set of tubes instead of 10-20.

[Tom-s]

How does this voltage variation affect the rest of the amplifier?

[troplin]

Yeah, I would go to 240V.

I've changed the wiring and now voltages are spot on.

Another question:
Does it matter if I mix tubes? The originals were RCA and the replacements Sylvania

[Paul Birkeland]

It's more important that the internal construction matches, but even this isn't critical.

-PB